An email validaor, from

BookAStar/BookAStar/Behaviors/EmailValidatorBehavior.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Text.RegularExpressions;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+
+namespace BookAStar.Behaviors
+{
+    public class EmailValidatorBehavior : Behavior<Entry>
+    {
+        const string emailRegex = @"^(?("")("".+?(?<!\\)""@)|(([0-9a-z]((\.(?!\.))|[-!#\$%&'\*\+/=\?\^`\{\}\|~\w])*)(?<=[0-9a-z])@))" +
+            @"(?(\[)(\[(\d{1,3}\.){3}\d{1,3}\])|(([0-9a-z][-\w]*[0-9a-z]*\.)+[a-z0-9][\-a-z0-9]{0,22}[a-z0-9]))$";
+
+        // Creating BindableProperties with Limited write access: http://iosapi.xamarin.com/index.aspx?link=M%3AXamarin.Forms.BindableObject.SetValue(Xamarin.Forms.BindablePropertyKey%2CSystem.Object) 
+
+        static readonly BindablePropertyKey IsValidPropertyKey = BindableProperty.CreateReadOnly("IsValid", typeof(bool), typeof(EmailValidatorBehavior), false);
+
+        public static readonly BindableProperty IsValidProperty = IsValidPropertyKey.BindableProperty;
+
+        public bool IsValid
+        {
+            get { return (bool)base.GetValue(IsValidProperty); }
+            private set { base.SetValue(IsValidPropertyKey, value); }
+        }
+
+        protected override void OnAttachedTo(Entry bindable)
+        {
+            bindable.TextChanged += HandleTextChanged;
+        }
+
+
+        void HandleTextChanged(object sender, TextChangedEventArgs e)
+        {
+            IsValid = (Regex.IsMatch(e.NewTextValue, emailRegex, RegexOptions.IgnoreCase, TimeSpan.FromMilliseconds(250)));
+            ((Entry)sender).TextColor = IsValid ? Color.Default : Color.Red;
+        }
+
+        protected override void OnDetachingFrom(Entry bindable)
+        {
+            bindable.TextChanged -= HandleTextChanged;
+
+        }
+    }
+}